Fabrication of ionic liquid electrodeposited cu-sn-zn-s-se thin films and method of making

ABSTRACT

A semiconductor thin-film and method for producing a semiconductor thin-films comprising a metallic salt, an ionic compound in a non-aqueous solution mixed with a solvent and processing the stacked layer in chalcogen that results in a CZTS/CZTSS thin films that may be deposited on a substrate is disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority from U.S. Provisional PatentApplication No. 61/581,962, filed Dec. 30, 2012; the subject matter ofwhich hereby is specifically incorporated herein by reference for allthat it discloses and teaches.

CONTRACTUAL ORIGIN

The United States Government has rights in this invention under ContractNo. DE-AC36-08GO28308 between the United States Department of Energy andthe Alliance for Sustainable Energy, LLC, the Manager and Operator ofthe National Renewable Energy Laboratory.

BACKGROUND

Photovoltaic (PV) solar electric technology will be a significantcontributor to world energy supplies when reliable, efficient PV powerproducts are manufactured in large volumes at low cost. A promisingpathway to reducing PV cost is the use of thin-film technologies inwhich thin layers of photoactive materials are deposited inexpensivelyon large-area substrates. The primary chalcogenide semiconductorabsorber materials currently used for thin-film PV device applicationsare Cu(In,Ga)Se₂ and CdTe. Despite the promise of these technologies,the toxicity of Cd and supply limitations for In and Te are projected tolimit the production capacity of these existing chalcogen-basedtechnologies to less than 100 Global Warming Potential (GWP) per year.This represents a small fraction of the world's growing energy needs,which are expected to double to 27 Terrawat by 2050.

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods that aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be considered illustrative rather than limiting.

FIG. 1 is a block diagram illustrating one embodiment of a Cu—Zn—Sn—S—Se(CZTSS) semiconductor thin film.

FIG. 2 illustrates X-ray diffraction patterns for several films withvarying atomic ratios.

FIG. 3 a illustrates dark and light current response curves of anelectrodeposited CZTSS device.

FIG. 3 b illustrates the external quantum efficiency (EQE) spectrum ofthe CZTSS device.

FIG. 4 illustrates the SEM results of the thin films. FIG. 4 aillustrates the SEM surface morphology and FIG. 4 b illustrates the SEMcross-section of annealed CZTSS thin films.

FIG. 5 is a flowchart of a method of forming chalcogen-basedsemiconductor thin films within an electrochemical device.

DETAILED DESCRIPTION

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thedrawings and by study of the following descriptions.

The inventors of the present application realized that combining the useof electrodeposition and ionic liquids in the fabrication ofchalcogen-based semiconductor thin films, such has copper, zinc, tin,sulfur and selenium (CZTSS), produced unexpected results that providealternatives to copper indium gallium diselenide (CIGS). Low-cost,non-vacuum electrodeposition of CZTSSe thin film from aqueous and alsoionic liquid electrolytes circumvents the toxicity typically associatedwith the use of hydrazine. Ionic liquids have wide electrodepositionpotential windows and high thermal stability, resulting in higherdeposition efficiencies for electronegative species and allowing higherdeposition temperatures to promote in-situ crystallization, therebyavoiding the need for post-deposition thermal annealing. This solventsystem permits depositing CTZSS that is free of impurities (includingoxides and hydroxides). In order to avoid the water reduction,electrodeposition may be performed using non-aqueous solvents with alarge electrochemical window like ionic liquids (ILs).

Disclosed herein are metallic salts that may be used in a solution for asemiconductor thin-film. In certain embodiments, the solvent comprisesethylene glycol mixed with the ionic compound, water, HCl or boric acid(H₃BO₃). FIG. 1 illustrates an electrochemical structure 100 having asubstrate 110, a three-layered thin film comprising a first layer 120deposited onto the substrate 110, a second layer 130 deposited onto thefirst layer 120 and a third layer 140 deposited onto the second layer130. In some embodiments the thin-film that is deposited on a substratecomprises at least one copper salt, tin salt, zinc salt, or anycombination thereof. In some embodiments the substrate 110 may compriseglass, chromium, molybdenum, silicon, silicon dioxide, aluminum oxide,sapphire, germanium, an alloy of silicon and germanium, indium phosphide(InP), glass coated with a Molybdenum film 115, or any combinationthereof.

Exemplary electrochemical structure 100 may comprise a metallic salt, achalcogen and an ionic compound in a non-aqueous solution mixed with asolvent within the first layer 120, second layer 130 and third layer140. In some embodiments the first layer 120 and second layer 130 maycomprise a metallic salt. The metallic salt may comprise copper sulfate,stannous chloride (SnCl₂), stannic chloride (SnCl₄), zinc sulfate, zincchloride or any combination thereof. In some embodiments, the chalcogenmay comprise selenium, sulfur, telluride, polonium and combinationsthereof. In other embodiments, the third layer 140 may comprise an ioniccompound in a non-aqueous solution mixed with a solvent such as cholinechloride-ethylene glycol solution or a sulfonate-based solution. Incertain embodiments, the sulfonate-based solution is mixed with asurfactant.

Suitable ionic compounds or supporting electrolyte/complexing ionscomprise salts such as sodium sulfate, sodium fluoride, potassiumfluoride, potassium bifluoride, sodium bifluoride, sodium chloride, zincchloride, potassium chloride, or choline chloride (C₅H₁₄ClNO). In someembodiments, where the ionic compound comprise sulfonate, it may bemixed with a surfactant. The sulfonate may comprise mesylate, triflate,tosylate, or besylate. Exemplary solvents for dissolving the ioniccompounds or supporting electrolyte/complexing ions are provided in theexamples, such as water or choline chloride mixed with ethylene glycol,HCl or boric acid, but any solvent that dissolves an ionic compound maybe used.

In certain embodiments, the plating current density is in the range ofapproximately 1 to 8 mA/cm². The pH of the solution may be in the rangeof approximately 1-5. This p-type CTZSS semiconductor has optical bandgap energy of about 1.2 eV with a tremendous potential for commercialproduction of low-cost, high-efficiency PV modules. All CTZSS materials(Cu—Sn—Zn—S—Se) are earth abundant and have a cell efficiency ofapproximately 20%.

FIG. 2 shows the X-ray diffraction (XRD) patterns of three annealed CZTSthin films (D1, D2 and D3). The precursor electrodeposited Cu/Sn/Znstacked layers were annealed in a tube furnace at 550° C. in elemental Satmosphere for 60 minutes. All samples were prepared at same conditionto check the reproducibility of the deposition conditions. The precursorfilm compositions of the films, as analyzed by ICP-MS, were Cu:47-49 at%; Zn:27-25 at %, and Sn:26-24 at %. As shown in FIG. 2, the XRDpatterns are almost identical for all three samples representingKesterite CZTS structure [joint committee on powder diffractionstandards (JCPDS) #26-0575].

FIGS. 3 a and 3 b show the device efficiency of 3.6% of an annealedelectrodeposited CZTSS film. As shown in FIG. 3 a, the dark and lightcurrent response curves of an electrodeposited CZTSS device. FIG. 3 bshows the external quantum efficiency (EQE) spectrum of a representativeCZTSS device. The device efficiency (FIG. 3 a) that resulted from theprocessed electrodeposited precursor film in Se and S atmosphere was3.6% with a respectable V_(oc) (0.54 V). FIG. 3 b displays the externalquantum efficiency (EQE) spectrum of a representative device. This EQEspectrum reveals that the optical band gap of the CZTSS thin film is˜1.55 eV (800 nm). To improve the device efficiency, one can optimizethe electrodeposition and processing conditions of the stacked layer inSe and S at high temperature.

The surface morphology and cross-sectional view (SEM) of arepresentative annealed film is shown in FIGS. 4 a and 4 b. The SEM asshown in FIG. 4 a and FIG. 4 b, indicate that films are crack-free andhave a compact dense morphology. The cross-sectional view (FIG. 4 b) ofthe film shows the film thickness is about 1.3 μm and it has a veryrough surface morphology. The grain size determined from the top-viewand cross-sectional images ranged from about 100 to 500 nm, and thegrains exhibit sharp facets. This result indicates that we need tofurther optimize the deposition and processing conditions to obtainsmooth and uniform films. Solar cell devices were fabricated from theseabsorber materials.

All depositions may be accomplished galvanostatically at rates ofapproximately 3-8 mA cm⁻² s⁻¹ for between approximately 40-180 seconds.The above stacked metal films may be sulfurized/selenized in a quartztube furnace. The thin films and approximately 200-800 mg of thechalcogen (sometimes both) may be inserted into the furnace underflowing nitrogen. The furnace may be ramped to approximately 400-600° C.in approximately 20-120 minutes, held at temperature for approximately20-40 minutes, and cooled slowly. In other embodiments, the furnace maybe ramped to about 500-600° C. in approximately 20-120 minutes, held attemperature for about 20-30 minutes, and cooled slowly. The annealedsamples may be built into devices. CdS may be deposited via a chemicalbath 150, on top of which a bilayer comprising ZnO may be deposited viaRF sputter 160, followed by an electron beam sputtering process of Ni/Alto form grids 170.

The annealed samples may be analyzed via X-ray diffraction (XRD). Theratio of Cu:Zn:Sn may be determined via ICP and X-ray fluorescence. Thecrystal structure of films with atomic ratios between 2.0:1.2:1.0 to1.2:0.8:1.0 reveal some differences, namely copper-poor films have muchstronger peaks, but also have peaks from other phases.

FIG. 5 illustrates one method 500 for fabricating an electrochemicaldevice with a three-tiered thin film layers comprising copper, zinc,tin, sulfur and selenium described herein. The method 500 begins at 510such as by designing of a particular device to be fabricated such aslithium-ion battery or the like. The method 500 continues with selectingmaterial for the substrate 520, first layer 521, second layer 522, andthird layer 523. At 530, the method 500 may include the depositionapplication of the three layers. The method 500 continues withdepositing the first layer onto the substrate 540, followed bydepositing the second layer onto the first layer 550 and depositing thethird layer onto the second layer 560. The device is completed byannealing the stacked electrodeposited layers 570 in chalcogen, followedby depositing CdS 580, RF sputtering of ZnO layers 581, depositing topcontacts 582 and depositing an antireflective coating 583. The method500 then may end at 590.

Exemplary methods for electrodepositing thin films are provided in theexamples, but any method suitable for moving metal ions in a solution byan electric field to coat an electrode may be used. An exemplaryembodiment may involve films electrodeposited by potentiostatic methods.

Further provided are methods for producing a thin-film on a substratecomprising a metallic salt, a chalcogen and an ionic compound in anon-aqueous solution mixed with a solvent. In some embodiments, theionic compound may comprise sodium sulfate, sodium fluoride, potassiumfluoride, potassium bifluoride, sodium bifluoride, sodium chloride, zincchloride, potassium chloride, or choline chloride (C₅H₁₄ClNO). In someembodiments, where the ionic compound comprises sulfonate, it may bemixed with a surfactant. The sulfonate may comprise mesylate, triflate,tosylate, or besylate.

EXAMPLES

The foregoing examples of the related art and limitations relatedtherewith are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the drawings.

Example #1

A first layer comprising copper may be electrodeposited on amolybdenum/chromium/glass substrate. Copper may be electrodeposited froman aqueous bath comprising CuSO₄ (100 mM) and Na₂SO₄ (100 mM). A secondlayer comprising tin may be electrodeposited from an aqueous bathcomprising SnCl₂ (150 mM), SnCl₄ (5-10 mM), NaF (800 mM), NaCl (450 mM)and KF (550 mM); HCl may be used to acidify the bath to a pH ofapproximately 5. A third layer comprising zinc may be electrodepositedfrom an aqueous bath comprising ZnSO₄ (100 mM), ZnCl₂ (100 mM), andH₃BO₃ (215 mM).

Example #2

A first layer comprising copper may be electrodeposited on amolybdenum/chromium/glass substrate. Copper may be electrodeposited froman aqueous bath of CuSO₄ (100 mM) and Na₂SO₄ (100 mM). A second layercomprising tin may be electrodeposited from an ionic liquid (IL)solution comprising choline chloride-ethylene glycol (ChCl:EG).Approximately 626 g of ChCl and 500 mL of EG may be heated atapproximately 90° C. to prepare the ionic liquid solution. A tin platingbath may be prepared by dissolving approximately 0.1 M SnCl₂ in ChCl:EGsolvent. A third layer comprising zinc may be electrodeposited from anaqueous bath comprising ZnSO₄ (100 mM), ZnCl₂ (100 mM), and H₃BO₃ (215mM).

Example #3

A first layer comprising copper may be electrodeposited onmolybdenum/chromium/glass substrate. Copper may be electrodeposited froman aqueous bath of CuSO₄ (100 mM) and Na₂SO₄ (100 mM). A second layercomprising tin may be electrodeposited from an ionic liquid (IL)solution comprising choline chloride-ethylene glycol (ChCl:EG). About626 g of ChCl and 500 mL of EG may be heated at about 90° C. to preparethe ionic liquid solution. A tin plating bath may be prepared bydissolving approximately 0.1 M SnCl₂ in ChCl:EG solvent. A third layercomprising zinc may be electrodeposited from an ionic liquid (IL)solution comprising choline chloride-ethylene glycol (ChCl:EG). About626 g of ChCl and 500 mL of EG may be heated at about 90° C. to preparethe ionic liquid solution. A Zinc plating bath may be prepared bydissolving approximately 0.1 M ZnCl₂ in ChCl:EG solvent.

Example #4

A first layer comprising copper may be electrodeposited on amolybdenum/chromium/glass substrate. Copper may be electrodeposited froman aqueous bath comprising CuSO₄ (100 mM) and Na₂SO₄ (100 mM). A secondlayer comprising tin may be electrodeposited from a sulfonate-basedelectrodeposition bath with surfactant (Empigen BB). Tin may be used asa counter electrode. The Faradic efficiency of anode reaction (i.e. Sndissolution) is as high as cathodic reaction (both close to 100%). Athird layer comprising zinc may be electrodeposited from an ionic liquid(IL) solution comprising choline chloride-ethylene glycol (ChCl:EG).About 626 g of ChCl and 500 mL of EG may be heated at about 90° C. toprepare the ionic liquid solution. Zinc plating bath is prepared bydissolving about 0.1 M ZnCl₂ in ChCl:EG solvent. Zn may be used as acounter electrode. The Faradic efficiency of anode reaction (i.e. Zndissolution) is as high as cathodic reaction (both close to 100%).

Example #5

Thin film Cu—Zn—Sn—(Se,S) was prepared by annealing the stackedelectrodeposited Cu/Sn/Zn layer in a tube furnace in the presence ofelemental sulfur and selenium at about 550° C. First a Cu layer waselectroplated on Mo/glass substrate from a solution containing 25 gmCuSO₄.5H₂O and 14 gm Na₂SO₄ dissolved in 1000 ml water. The pH of thesolution was adjusted to 1.65 by adding 2.6 ml H₂SO₄. Second Sn layerwas deposited on Cu/Mo/glass from a solution containing 17.5 SnCl₂.2H₂Odissolved in 500 ml ionic liquid solvent. Ionic liquid solvent wasprepared by dissolving 313 gm choline chloride in 500 ml ethyleneglycol. Third Zn layer was prepared from 90 gm ZnSO₄.4H₂O, 7.5 gm ZnCl₂,6.2 gm H₃BO₃ and 5 ml Epigen BB dissolved in 500 ml water.

Photovoltaic devices were completed by chemical-bath deposition of about50 nm CdS, followed by RF sputtering of 60 nm of intrinsic ZnO and 120nm of Al₂O₃-doped conducting ZnO. Bilayer Ni (50 nm)/Al (3 μm) topcontacts were deposited in an e-beam system. The final step in thefabrication sequence was the deposition of an antireflection coating(100 nm of MgF₂). The current-voltage (I-V) of the devices is shown inFIG. 2. The cell parameters are 0.54 V open-circuit voltage, 16.9 mA/cm²short-circuit current density, 40% fill factor, and 3.6% efficiency. Thedevice efficiency before antireflection coating MgF₂ is 3.4%(open-circuit voltage: 0.53 V, short-circuit current density: 16.1mA/cm², fill factor: 40%).

Example #6

Electrodeposition of Cu—Sn—Zn was performed sequentially from aCu-plating solution, Sn-plating solution, and Zn-plating solution,respectively. First, a Cu layer was electrodeposited on a Mo/glasssubstrate from a Cu-plating solution, the second Sn layer waselectrodeposited from a Sn-plating solution, and the third Zn layer waselectrodeposited from a Zn-plating solution. The solution concentrationsof each deposition solutions were 0.1 M. Fisher Scientific (FB300) andVWR (300V) power supplies were used to electrodeposit Cu—Sn—Zn thinfilms. All films were electrodeposited by applying constant current. Cuwas plated at −4.2 mA/cm² for 3 minutes, Sn was plated at −2.0 mA/cm²for 22 minutes and Zn was plated at −1.7 mA/cm² for 4 minutes.

The desired film composition was obtained by adjusting the filmthickness of Cu, Sn and Zn. The films were electrodeposited in avertical cell in which the electrodes (both working and counter) weresuspended vertically from the top of the cell. Precursor films wereprepared by employing a two-electrode cell in which the counterelectrode was Pt gauze and the working electrode (substrate) wasglass/Mo. The Mo film was about 1 μm thick and was deposited by directcurrent (dc) sputtering. All chemicals were of Analar- orPuratronic-grade purity and were used as received. The film compositionswere analyzed using Agilent Technologies 7700 Series ICP-MS system.X-ray diffraction (XRD) was performed by a Scintag X-ray machine using aCopper K_(α1) radiation at λ=0.5456 Å. PV devices were completed bychemical-bath deposition of about 50 nm CdS, followed by radio frequency(RF) sputtering of 50 nm of intrinsic ZnO and 350 nm of Al₂O₃-dopedconducting ZnO. Bilayer Ni/Al top contacts were deposited in an e-beamsystem.

The Examples discussed above are provided for purposes of illustrationand are not intended to be limiting. Still other embodiments andmodifications are also contemplated. While a number of exemplary aspectsand embodiments have been discussed above, those of skill in the artwill recognize certain modifications, permutations, additions and subcombinations thereof. It is therefore intended that the followingappended claims and claims hereafter introduced are interpreted toinclude all such modifications, permutations, additions andsub-combinations as are within their true spirit and scope.

We claim:
 1. A semiconductor thin-film comprising: a substrate; a firstlayer deposited onto the substrate, comprising a metallic salt; a secondlayer deposited onto the first layer, comprising a metallic salt; and athird layer deposited onto the second layer, comprising an ioniccompound in a non-aqueous solution mixed with a solvent, wherein thefirst, second and third layers are stacked and annealed in chalcogen. 2.The semiconductor thin-film according to claim 1, wherein the metallicsalt consists of a group selected from copper sulfate, stannous chloride(SnCl₂), stannic chloride (SnCl₄), zinc sulfate, or zinc chloride. 3.The semiconductor thin-film according to claim 1, wherein the chalcogenis selected from a group consisting of selenium, sulfur, telluride orpolonium.
 4. The semiconductor thin-film according to claim 1, whereinthe ionic compound is selected from a group consisting of sodiumsulfate, sodium fluoride, potassium fluoride, potassium bifluoride,sodium bifluoride, sodium chloride, zinc chloride, or potassiumchloride.
 5. The semiconductor thin-film according to claim 1, whereinthe ionic compound comprises choline chloride (C₅H₁₄ClNO).
 6. Thesemiconductor thin-film according to claim 1, wherein the solventcomprises ethylene glycol mixed with the ionic compound.
 7. Thesemiconductor thin-film according to claim 1, wherein the solventcomprises water, HCl or boric acid (H₃BO₃).
 8. The semiconductorthin-film according to claim 1, wherein the ionic compound comprisessulfonate.
 9. The semiconductor thin-film according to claim 1, whereinthe solvent comprises a surfactant mixed with the ionic compound. 10.The semiconductor thin-film according to claim 1, wherein the sulfonateis selected from a group consisting of mesylate, triflate, tosylate, orbesylate.
 11. A method for producing a semiconductor thin-filmcomprising the steps of: (a) electrodepositing a first layer of ametallic salt from an aqueous bath onto a substrate; (b)electrodepositing a second layer comprising a metallic salt onto thefirst layer; (c) electrodepositing a third layer comprising an ioniccompound in a non-aqueous solution mixed with a solvent onto the secondlayer; and (d) annealing the first, second and third stacked layers inchalcogen.
 12. The method of claim 11, wherein the metallic salt isselected from a group consisting of copper sulfate, stannous chloride(SnCl₂), stannic chloride (SnCl₄), zinc sulfate, or zinc chloride. 13.The method of claim 11, wherein the chalcogen is selected from a groupconsisting of selenium, sulfur, telluride or polonium.
 14. The method ofclaim 11, wherein the ionic compound is selected from a group consistingof sodium sulfate, sodium fluoride, potassium fluoride, potassiumbifluoride, sodium bifluoride, sodium chloride, zinc chloride, orpotassium chloride.
 15. The method of claim 11, wherein the ioniccompound comprises choline chloride (C₅H₁₄ClNO).
 16. The method of claim15, wherein the solvent comprises ethylene glycol mixed with the ioniccompound.
 17. The method of claim 11, wherein the solvent compriseswater, HCl or boric acid (H₃BO₃).
 18. The method of claim 11, whereinthe ionic compound comprises sulfonate.
 19. The method of claim 18,wherein the solvent comprises a surfactant mixed with the ioniccompound.
 20. The method of claim 18, wherein the sulfonate is selectedfrom a group consisting of mesylate, triflate, tosylate, or besylate.21. The method of claim 11, wherein the semiconductor thin-film isdeposited on a substrate comprising at least one copper salt.
 22. Themethod of claim 11, wherein the semiconductor thin-film is deposited ona substrate comprising at least one tin salt.
 23. The method of claim11, wherein the semiconductor thin-film is deposited on a substratecomprising at least one zinc salt.
 24. The method of claim 11, furthercomprising a plating current density in the range of approximately 1 to8 mA/cm².
 25. The method of claim 11, wherein the substrate is selectedfrom a group consisting of glass, chromium, molybdenum, silicon, silicondioxide, aluminum oxide, sapphire, germanium, an alloy of silicon andgermanium, indium phosphide (InP) or any combination thereof.
 26. Themethod of claim 11, wherein the substrate comprises glass coated with aMolybdenum film.
 27. The method of claim 11, wherein the electroplatingsolution comprises a pH in the range of approximately 1-5.